This invention relates generally to borosilicate glass compositions particularly suitable for sealing to nickel iron metal alloys to provide a direct hermetic seal exhibiting thermal endurance. More particularly, the present glass compositions can be reliably sealed directly to commercial nickel iron alloys now being used as the lead-in conductors for electric lamps wherein the resulting hermetic seal will be subjected to severe thermal shock conditions when the lamp is operated. All glass type flash lamps fit into this product category by reason of requiring a transparent glass envelope which is hermetically sealed directly to a pair of inlead conductors since the flash lamps change temperature rapidly from room temperature to near the glass softening point in a few milliseconds when ignited.
Kovar metal alloys have been used for some time to provide glass-to-metal seals in flash lamps because of particular thermal expansion characteristics which can create compression in seals of said metal alloys to glass. The general range of composition for said Kovar alloys is about 27-32% nickel, 14-19% cobalt, less than 1% manganese, an the balance iron by weight except for incidental impurities. A known borosilicate glass found to be especially suitable for this type glass-to-metal seal is disclosed in U.S. Pat. No. 3,506,385 assigned to the present assignee. The composition of this known glass is in the range by weight percent: 60-75 SiO.sub.2, 10-25 B.sub.2 O.sub.3, 1-10 Al.sub.2 O.sub.3, 4-10% total alkali oxides and 0-5% BaO except for incidental impurities, residual fluxes and refining agents with said glass composition exhibiting a mean coefficient of linear thermal expansion between 0.degree. and 300.degree. C. in the range 40-50.times.10.sup.-7 per .degree.C. While a commercial nickel iron alloy is known which contains around 42% nickel and exhibits a thermal expansion characteristic similar to said Kovar metal alloys, it has not been found possible to reliably seal inleads of said nickel iron alloy with the prior art glass composition during lamp manufacture. The glass-to-metal seals produced in this manner have exhibited high stress levels leading to unsatisfactory containment of the flash lamp reaction and breakage of the lamps from thermal shock.
Other thermal expansion characteristics for nickel iron alloys containing about 40-45 weight percent nickel as compared with Kovar-type alloys require that modified borosilicate glass composition be found for the glass-to-metal seals if subjected thereafter to significant thermal shock conditions. Specifically, these nickel iron alloys undergo greater thermal expansion above 350.degree. C. than is experienced by the Kovar-type alloy although exhibiting a thermal expansion coefficient in the 0.degree.-350.degree. C. range of about 45-56.times.10.sup.-7 per .degree.C. which closely approximates that for the Kovar alloy. By lowering the transformation temperature of the prior art glass composition, however, plastic flow in the glass could begin at a lower temperature to relieve the physical stress caused by thermal shock. It is an object of the present invention, therefore, to provide a superior borosilicate glass for direct sealing to the nickel iron alloys above defined having improved thermal shock resistance.